1. Field of the Invention
The present invention relates to an oxygen concentration sensor for detecting oxygen concentration, for example, in exhaust gas from a vehicle engine. More specifically, the present invention relates to a method for detecting element resistance based on voltage and current frequency characteristics of the oxygen concentration sensor.
2. Description of Related Art
In recent years, with regards to the control of air-fuel ratio of a vehicle engine, there have been demands for improving control precision, for promoting lean burning and the like. To meet such demands, a linear type air-fuel ratio sensor (oxygen concentration sensor) for linearly detecting the air-fuel ratio (that is, oxygen content of exhaust gas) of an air mixture drawn into the engine has been proposed and implemented. In order to maintain its precise level of detection, the air-fuel ratio sensor of the above-mentioned type must be kept in an active state. In general, this active state is maintained by heating a sensor element of the sensor by controlling the actuation of a heater attached to the sensor.
According to such actuation control of the heater, the temperature of the sensor element (hereinafter simply referred to as "element temperature") is detected, and feedback control is subsequently performed so that the element temperature ultimately reaches a desired activation temperature (e.g., about 700.degree. C.). In this case, although the element temperature may be detected on a real-time basis based on a result detected by a temperature sensor attached to the sensor element, the attachment of the temperature sensor is likely to raise the overall cost of the sensor device. Because of this, there have been proposals to detect the resistance of the sensor element (hereinafter simply referred to as "element resistance") based on a predetermined relationship between element resistance and the element temperature, and thus, derive the element temperature from the detected element resistance. It is to be noted that the detected element resistance can also be used for determining, for example, the level of deterioration in the characteristics of the sensor.
FIGS. 53A and 53B are graphs that show a conventional procedure for detecting the element resistance as, for example, disclosed in U.S. Pat. No. 4,543,176. These figures illustrate a case in which a limit current type oxygen concentration sensor is used as an air-fuel ratio sensor for performing engine control. As shown in FIG. 53A, before a time instant t11, a predetermined voltage (positive voltage Vpos) for detecting the air-fuel ratio is applied to the sensor element. The air-fuel ratio is obtained based on a sensor current Ipos generated in accordance with the applied voltage Vpos as shown in FIG. 53B. Between time t11 and t12, a negative voltage Vneg for detecting element resistance is applied and the corresponding sensor current Ineg is detected. Then, the negative voltage Vneg is divided by the sensor current Ineg to obtain element resistance ZDC (ZDC=Vneg/Ineg). This method is generally known as a method for detecting element resistance based on DC characteristics of the air-fuel ratio sensor.
Although the above conventional method is used for detecting element resistance (the direct current component of impedance) by applying DC voltage to the sensor element, U.S. Pat. No. 4,419,190 discloses a method for detecting element resistance through the application of an alternating current (AC) voltage to the sensor element. In this method, alternating current is continuously applied to the air-fuel ratio sensor and the sensor output from the air-fuel ratio sensor is passed through a low-pass filter (hereinafter referred to as LPF) to detect the air-fuel ratio. The same sensor output is passed through a high-pass filter (hereinafter referred to as HPF) and averaged to detect alternating current impedance. This method is generally known as a method for detecting element resistance using AC characteristics of the air-fuel ratio sensor.
However, all the conventional methods described above have the following problems. Namely, according to the DC impedance method, when a negative voltage Vneg having a rectangular waveform is applied, the sensor current Ineg changes rapidly and thus, the peak current of the sensor cannot be detected accurately. For this reason, the detection of the peak current has to be discontinued until the sensor current stabilizes. Accordingly, there will be a period of time during which the air-fuel ratio cannot be detected. Furthermore, this problem of being unable to detect the peak current during the application of the voltage having a rectangular waveform will likely occur also in the method of detecting element resistance based on AC characteristics.
With the AC impedance method (disclosed in U.S. Pat. No. 4,419,190), because the sensor output is passed through the LPF to detect the air-fuel ratio, problems in the air-fuel ratio output such as phase shift and AC noise may occur. These problems are particularly evident when the operating condition of the engine is in a transition.